A career naval officer, Yonai was a  conservative anti-militarist. After resigning as premier he resumed his post as Navy minister under TOJO. He was instrumental in the construction of the modern Japanese navy.

After Guadalcanal he secretly advocated a negotiated peace.

After TOJOs removal in July 1944 Yonai declined the premiership buy agreed to Navy minister. Yonai had a decisive role on the Supreme Council for the Direction of the War in convincing the council to accept surrender.